70 million Americans suffer from some sort of sleep disorder. Behavior, mood and memory deteriorate with sleep loss and it gets worse with continuing sleep deprivation. Lack of sleep has been linked to Alzheimer?s disease. There is considerable amount of data on the neurons that keep us awake (arousal neurons) whereas very little is known about the neurons that make us fall asleep. Indeed, current network models of sleep-wake regulation list many arousal neuronal populations compared to only one sleep group located in the preoptic area. Better hypnotics will emerge by identifying circuits that directly link specific phenotypes of neurons to sleep. To provide this evidence, we will use the deep-brain imaging method to visualize activity of phenotype specific neurons in the hypothalamus, a part of the brain where neurons regulating sleep and arousal are located. Aim 1 will image neurons containing melanin concentrating hormone (MCH) based on our evidence published in the last funding period that optogenetic stimulation of these neurons robustly increases both non-REM and REM sleep in mice and rats. Aim 2 will use new retrograde axon-track tracing tools to insert ChR2 into projection specific MCH neurons to mechanistically demonstrate that driving specific circuits induces sleep. The CLARITY method will be used to visualize the circuit. In Aim 3, optogenetics will drive these circuits to induce sleep in conditions of high arousal. Solid preliminary data are presented for all aims indicating that these methods are operational in the lab, and that the project will be completed successfully. From a translational perspective these studies are potentially useful in sleep disorders, such as insomnia, where sleep needs to be triggered against a strong arousal drive. These aims will provide a framework for integrating the sleep-active neurons within an overall model of sleep-wake regulation.